The frequency of a repetitive signal has typically been measured using one of two methods The most commonly used method is to set up a time interval and count the number of pulses which occur during that interval. A disadvantage of this method is that when the frequency is low and only a few pulses occur during the time interval, the accuracy is low since one cannot resolve partial pulses. One solution to this problem is to increase the time interval so that many pulses occur during the time interval regardless of the frequency. This frequency measuring method is not suitable, however, when used in a system which requires a rapid update of the measured frequency.
A second frequency measuring method measures the time between pulses (period) and converts this measurement into a frequency. This method overcomes the disadvantage associated with the method described above but poses another disadvantage. When the frequency is high, the time between pulses must be measured to a high degree of accuracy. This is not always possible because the method used to measure time cannot always be that accurate For example, when working with a microprocessor using a 1 MHz. clock frequency, the most accurately one can measure a period is to 1 microsecond. Often this is not accurate enough when measuring frequencies above 1 Khz.
U.S. Pat. No. 4,052,620 to Brunnett discloses a variation of a constant time interval frequency determination method. That patent shows apparatus for conducting a computed tomography (CT) scan of a patient An x-ray intensity is determined by converting an analog electrical output from radiation photo detectors into a series of pulses whose frequency is proportional to the radiation intensity.
The Brunnett apparatus establishes a primary time Period based on the scan time of the scanner During this primary period, a variable frequency is generated which corresponds to an analog signal from the photodetectors What is desired is the average value of the signal during the primary period of the scan. A secondary period is generated which starts on the first pulse after the primary period starts and ends on the last pulse before the primary period ends. The actual frequency of the pulses at any one time is of no interest, only the average during the primary period. The secondary period is the largest period available which can be synchronized to the pulses and will fit inside of the primary period.